1. Field of the Invention
The field of the invention is that of positioning (i.e. determining the geographical location of) mobile stations using radiolocation techniques. These techniques enable a mobile station to determine its geographical location at any time using radio links between the mobile station and one or more reference points.
2. Description of the Prior Art
Many positioning systems exist already. They are essentially intended for maritime and aeronautical applications and use a specific transmission infrastructure such as a network of terrestrial transmitters (as in the OMEGA or LORAN C systems, for example) or a constellation of satellites (as in the TRANSIT system, for example).
The existing Global Positioning System (GPS) can be used in any type of mobile station. It uses a constellation of 24 satellites deployed in such a manner that a mobile station can receive signals from three separate satellites at any time. In the GPS system three satellites are required for positioning to be possible.
A major drawback of these prior art dedicated systems is that they require a very high investment in terms of transmission network installation and maintenance.
These infrastructure costs also impact on the positioning devices. Furthermore, these devices must include powerful and accurate computation means since they have to achieve an accuracy in the order of a few tens of meters.
This degree of accuracy is of benefit in some applications. There is, nevertheless, an entirely separate requirement for coarser positioning (for example, at the level of one cell in a cellular mobile radio network) at much lower cost. At present there is no system meeting this need.
One object of the invention is to meet this need and to alleviate the various drawbacks of the prior art.
To be more precise, one object of the invention is to provide a positioning method using a simplified satellite infrastructure. In other words, one object of the invention is to provide a positioning method requiring reception of signals from only one satellite (rather than from three satellites, as in the prior art systems).
Another object of the invention is to provide a method of this kind such that the design and manufacture of the positioning devices combine simplicity with low cost. In particular, one object of the invention is to provide a positioning method which can be implemented easily in mobile telephones.
A further object of the invention is to provide a positioning method which does not require a specific infrastructure but which can use an existing infrastructure such as the Globalstar network, for example.